I don't think YABU but I don't think they're necessarily being rude either. They're probably curious, but clumsy.
Many sighted people have a fear of blindness that probably people who've lived with partial or absence of sight their whole lives, or like my MIL, have developed it in later life, don't have, or at least they don't consider to be the end of the world by a long chalk.
sofia and maidofstars are probably like me. I'm professionally nosy because I'm a journalist. I'm sure I upset people all the time with dumb questions.
Sometimes to avoid all misconception I have to ask really simple questions. If you put me in my place, I'll accept it, as long as you're nice about it 
I wouldn't have asked the sex question though 
It's an old one, but my favourite Woody Allen quote is that if reincarnation exists, he wants to come back as Warren Beatty's fingertips. 
I don't know if you're old enough to remember when Warren Beatty was a gorgeous shagger. Or who the modern equivalent would be.